The present invention relates to non-ionic photo-acid generating polymers for resist applications, and more specifically, to non-ionic photo-acid generating polymers for extreme ultraviolet lithography bearing pendant sulfonate esters of N-hydroxy imides.
Extreme Ultraviolet (EUV) lithography is expected to succeed current 193 nm immersion lithography combined with multiple exposure enhancements as the next generation printing technique for sub-40 nm features. EUV radiation, with a shorter wavelength of 13.5 nm, is expected to achieve finer features without the need for multiple exposures. However, more advances in more powerful light sources, EUV masks, and resists are needed for commercialization of EUV lithography.
In recent years, considerable effort has gone into the development of resists for EUV applications. However, a majority of EUV resists are modified resists for 193 nm and 248 nm lithography.
The most efficient resists for 193 nm and 248 nm lithography are based on chemical amplification, where an initial relatively low dose of radiation induces formation of a catalyst, often a strong acid. Each molecule of the catalyst induces a relatively large number of chemical events, thereby chemically amplifying the effect of the exposure. For example, chemical amplification of a positive tone resist can involve converting aqueous base insoluble groups of exposed areas to aqueous base soluble groups. In the case of negative tone resists, chemical amplification can involve crosslinking reactions in the exposed areas.
Currently favored positive resist compositions comprise acid-labile groups (e.g., carboxylic esters of tertiary alcohols). Prior to exposure, a layer comprising the resist composition is insoluble in aqueous base developer. During exposure to radiation, a photo-acid generator (PAG), which is present in the layer, produces strong acid. The photogenerated acid catalyzes the deprotection of the acid-labile groups of the exposed portions of the layer during a post exposure bake (PEB). The PEB renders exposed portions of the layer soluble in an aqueous base developer, allowing formation of a positive tone image.
A drawback of resists for 248 nm, 193 nm and E-beam applications is referred to as “image blur” (see, e.g., Hinsberg et al., Proc. SPIE, (2000), 3999, 148). Image blur is generally thought to result from two contributing factors: gradient-driven acid diffusion and reaction propagation, the result being a distortion in the developable image compared to the projected aerial image transferred onto the film. This becomes critical in EUV applications because of the need for very small features with low line edge roughness (LER).
Polymers comprising covalently bound photo-acid generator moieties (i.e., polymer-bound PAGs) were developed to control the gradient-driven acid diffusion. Many current EUV resist compositions now include a polymer-bound PAG, in which the acid functional groups generated during exposure are covalently linked to the polymer.
Many widely reported polymers comprising bound PAG groups for EUV applications are ionic in nature. The PAG moiety comprises a negative charged sulfonate group (*—SO3−1), which is covalently linked to the polymer, and a positive charged counter ion (e.g., triphenylsulfonium, iodonium, diphenyliodonium). U.S. Pat. No. 8,057,985 B2 to Ohashi, et al., discloses examples of polymer-bound ionic PAG groups. These type of polymers have limitations due to low incorporation of PAG bound repeating units (<5 mol %), higher polydispersity, and poorer solubility in casting solvents. These limitations prevent further improvements in resolution and LER using PAG-bound polymers.
A limited number of non-ionic PAG-bound polymers have been reported (U.S. Pat. No. 7,812,105 B2, U.S. Pat. No. 8,329,377 B2, US20120328985 A1). However, the thermal stability of these polymers and the ability to achieve high resolution (sub-40 nm) lithographic performance of formulations comprising these polymers have not been reported.
Another issue that affects the performance of the current EUV resists is their sensitivity to out-of-band radiation (150-400 nm) that is emitted by the light source along with the desired 13.5 nm radiation. The sensitivity of these resists to out-of-band (OOB) radiation is due to the fact that the resists were modified from 193 nm and 248 nm resists that have onium salt PAGs. The effect of OOB radiation is exemplified by the resist thickness loss that results in profile degradation.
Therefore, there is a need to develop high resolution EUV resists that are less sensitive to OOB radiation and provide improved LER.